Sensing a mass deposited onto a surface of a piezoelectric resonator is a technique that has been used in the measuring and testing field for decades. A conventional quartz crystal microbalance (QCM) typically includes a piezoelectric resonator capable of sensing loads less than a microgram. For small amounts of mass, a change in the frequency of a piezoelectric resonator is proportional to a mass change. Thus, QCM's have been used in a variety of applications, such as detectors for measuring humidity or the presence of other adsorbed gases, and as sensors for monitoring film thickness in thin-film deposition processes, to name just a few.
In the past, QCM sensors were generally designed to operate in air or other gaseous environments. More recently, QCM sensors have been designed to operate in liquids. For instance, the following article describes a specific application of an acoustic sensor having a quartz crystal resonator that is designed to operate in oil: Hammond et al., AN ACOUSTIC AUTOMOTIVE ENGINE OIL QUALITY SENSOR, Proceedings of the 1997 IEEE International Frequency Control Symposium, IEEE Catalog No. 97CH36016, pp. 72-80, 28-30 May 1997.
The Hammond et al. article notes that the viscosity of oil in an automobile is perhaps the single most important technical parameter of a modern crankcase lubricant. Thus, Hammond et al. propose an onboard sensor for measuring viscosity changes of crankcase oil in an automobile or other similar mechanisms. They describe a technique of measuring the viscosity of oil by operating an AT-cut quartz resonator immersed in the oil The sensor includes a drive circuit that excites a shear mechanical motion in the resonator, which motion is transferred to the oil. The oil essentially acts as a mechanical load to the quartz resonator and this mechanical load does influence the quality factor (Q) and other electrical properties of the resonator. The Hammond et al. article describes how a change in the electromechanical quality factor Q of a resonator is proportional to the mass accumulation at the resonator-oil interface. As such, changes in the resonant frequency and the amplitude of the resonance signal, due to the mechanical loading, are found to be proportional to the product of the density and viscosity of the oil. Thus, the combined effects of phase and amplitude changes of a sensing signal may be measured to monitor changes in the oil viscosity.
Others have used similar techniques to measure the properties of a variety of different liquids. The following articles describe resonator sensors capable of making simultaneous measurements of liquid density and viscosity: Zhang et al., CONTRIBUTIONS OF AMPLITUDE MEASUREMENT IN QCM SENSORS, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 43, No. 5, pp. 942-947, September 1996; and Martin et al., MEASURING LIQUID PROPERTIES WITH SMOOTH- AND TEXTURED-SURFACE RESONATORS, 1993 IEEE International Frequency Control Symposium, IEEE Catalog No. 0-7803-0905-7/93, pp. 603-608, 1993.
The Zhang et al. article describes how a QCM, having an AT-cut quartz resonator, can detect changes in viscosity and density of a liquid. This article indicates that when a QCM operates in a liquid, the total frequency change consists of two effects, one due to mass loading and the other due to "liquid damping." Further, according to Zhang et al., one cannot distinguish the mass loading effect from the total frequency change by only frequency measurement. Thus, a standard technique of using a QCM in liquids is to simultaneously measure changes in a frequency and a quality factor Q (or changes in equivalent circuit parameters). This allows separation of the mass loading effect from the liquid damping effect.
The Martin et al. article describes an improved method that uses a dual-resonator sensor with two AT-cut quartz resonators, one with a smooth surface and the other with a textured or rough surface. The surface texture comprises ridges, which are oriented perpendicular to the direction of surface shear displacement, i.e., the X crystalline direction. When operated in a liquid, the smooth resonator generates plane-parallel laminar flow in the adjacent liquid, which causes a resonator frequency shift that is a function of liquid density and viscosity. A textured resonator, however, traps a quantity of liquid in excess of that entrained by a smooth surface. The trapped liquid behaves as an ideal mass layer, causing an additional frequency shift that depends only on density and not viscosity.
In the Martin et al. sensor, each resonator is driven by an independent oscillator circuit that provides the following two outputs: a radio frequency (RF) signal that tracks resonant frequency and a direct current (DC) voltage proportional to motional resistance. Baseline responses are determined by measuring resonant frequency and motional resistance for each resonator before their immersion in a liquid. Changes in resonator responses are then measured separately for the smooth and textured resonators after immersion. A computer connected to the sensor calculates density and viscosity. In particular, the liquid density is first calculated from the difference in responses measured between the smooth and textured devices. Having determined liquid density, the response of the smooth resonator is then used to calculate liquid viscosity. Thus, the Martin et al. method measures a frequency change and a quality factor (Q) change (or a change in equivalent circuit parameters) for each resonator separately.
Although standard techniques of sensing the properties of fluids have served the purpose, they have not proved entirely satisfactory when making highly sensitive measurements of fluid properties, including viscosity and density. Sensor designers acknowledge that while changes in frequency can be measured with great accuracy, changes in Q, motional resistance or any other quantity are normally measured with significantly less accuracy. Q measurements for high-Q devices are typically made with accuracies of two to four significant figures, whereas the frequencies of stable frequency sources can be measured with accuracies of 14 significant figures. For lower Q devices, such as resonators immersed in a fluid, both the Q and frequency measurement accuracies are lower, however, the frequency measurement accuracies are still orders of magnitude higher than the Q measurement accuracies.
Sensor fabricators have also recognized problems with using resonators with smooth and textured surfaces. Changes in frequency and Q depend not only on a liquid's properties, but also on a resonator's surface roughness. However, it is difficult to produce surfaces of identical surface roughness, i.e., it is difficult to produce a standard rough surface. An additional difficulty with the prior art is that temperature can greatly affect the properties of fluids, e.g., the fluid's viscosity. It is well known that, for example, the viscosity of many oils and lubricants vary with temperature, and also with degradations due to chemical changes. Measuring frequency and Q changes alone cannot determine the temperature of the fluid simultaneously with the fluid's viscosity and density. Therefore, when only frequency and Q are measured and a viscosity change is detected, it is not possible to determine the cause of the viscosity change; it could be due to a temperature change or to a change in the quality of the fluid, or to a combination of factors.